With a chance to beat a crippled-up opponent who couldn’t even snap the ball properly, the Redskins managed to out-ineffectual the other side. That falls squarely on their on-field leader.

On the biggest night of his rejuvenated career, when he most needed a good performance, Grossman brought Bad Rex instead of Good Rex. There’s no sweetening that tea. Grossman is a large part of why the Redskins are 2-1 with a fighting chance in the NFC East, but he bears disproportionate responsibility for a critical loss, too.

The fourth quarter came down to the heart-in-the-throat issue of whether they could survive the uneven play of their two-sided, double-edged quarterback. Could they beat the clock before he turned into Truly Awful Horrible Rex? The answer was no; he committed a catastrophic fumble with 28 seconds left.

To beat the Cowboys, the Redskins needed to put together one more moderately decent drive at any point in the game — just a few more grudging yards, here or there. A team relies heavily on the quarterback to make that happen, to make a play with his arm or his smarts or his legs, instead of a killing mistake. Grossman’s official stats reflect one touchdown pass, with one interception and one fumble, but the raw eyewitness data showed he was lucky not to be intercepted twice more, and he just didn’t scan the field well. The last four possessions of the game resulted in three punts and the fumble.

There were plenty of other culprits, from DeAngelo Hall to Sav Rocca to Jim Haslett. Kyle Shanahan bears significant responsibility for converting just 3 of 12 third downs.

“You want me to go on and on about it?” Grossman said. “I don’t think it is one main thing. Collectively, it’s not letting one play stall your drive.”

But the fact remains that the difference between 3-0 and 2-1 came down to a single game-ending blunder by the quarterback. As Grossman himself said, “We felt like we were moving into good position with 48 seconds left, needed about 25 yards to put us in decent range,” when he fumbled.

From the end of last season to the start of this season, we’ve now had a half-dozen regular-season games to judge Grossman. No question, he’s an upgrade over the paralyzing Donovan McNabb, and he’s not nearly the Train Rex his critics from his days in Chicago claim he is. He’s a better than adequate leader, with good study habits and a fine passing arm, currently the NFL’s 16th-ranked performer at his position.

“I don’t really judge Rex on what he did in Chicago, I judge him on what we ask him to do here and how he practices,” Coach Mike Shanahan said last week. “. . . and Rex has been very consistent since I’ve been here in the way he prepares and the way he works.”

But the difference between a merely decent team and a dynamic one is not whether a quarterback is adequate; it’s whether he’s elite. John Clayton of ESPN once did an intriguing calculation that showed teams with so-called “elite” quarterbacks beat teams with non-elite performers roughly 80 percent of the time. “Elite” means someone who completes 60 percent of his passes, can throw for 4,000 yards in a season, and has fourth-quarter-comeback ability. The Shanahans are gambling heavily they can transform Grossman into elite.

So is Grossman capable of becoming elite? He already has met two of the criteria: He’s completing 59.6 percent of his passes and averaging 282 yards a game, on pace to exceed 4,000, and he has led a team to a Super Bowl. There is no reason — absolutely none — that Grossman can’t kill off Bad Rex once and for all, and make game-winning plays consistently under pressure.

He can start the transformation today by going on a diet. He needs to lose weight, as he admits — and this is not a trivial point. If Grossman has an inherent limitation, it’s his lack of mobility. He has no business compounding it with a knife and fork if he expects to lead the Redskins to the playoffs.

He said of his final snap against the Cowboys, “I wanted to try to make a play, I thought I could slide and get it to Santana” Moss. But it’s pretty hard to extend a play with your legs carrying 10 extra pounds of fat: Anthony Spencer ran him down from behind like a pigeon. Also, it sends a bad message. He’s the one guy left in the Redskins’ locker room who looks anything other than hungrily lean.

Ball security is another totally fixable weakness. There are plenty of elite quarterbacks who lack mobility and who take sacks, starting with Tom Brady. They don’t fumble the way Grossman does. It’s a matter of technique, and disciplined repetition. It’s not Grossman’s fate to be a fumbler. He has some choice in the matter.

Good Rex gives the Redskins real firepower. With him under center, they are moving the ball between the 20s better than in years. Against Arizona they saw the red zone seven times. But they’ve left at least two un-scored touchdowns on the field in each of their three games.

They could be killing people. Grossman’s imperfections are not graven into his soul or character. The question is not can he fix them, but will he.

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